In 1923 the bacteriologists WWC Topley and GS Wilson from the University of Manchester coined the sentence Herd immunity in a study of how bacterial infections spread through a group of mice when some were vaccinated [PDF]. While they stressed the need for more research on the subject, the basic idea was that if a certain fraction of its individual members were immune, a community could potentially achieve comprehensive protection from an infectious disease.
For example, if a sick person came in contact with five people – four of whom were immune to the disease – that person could only pass it on to the one susceptible person. Not only would the other four people stay healthy, but they would also not pass the disease on to everyone they met. That way, individual cases are much easier to contain and a highly infectious disease becomes a much smaller threat to an entire community.
Almost a century after Topley and Wilson introduced the term, people have seen the effects of herd immunity in real life more than a couple of times. As the Association of Infection Control and Epidemiology Professionals explains, this often occurs when the majority of the population is vaccinated against a particular disease. It̵
Herd immunity can also occur when many people have already had a disease and now have antibodies that make them immune to re-infection. Since we don’t have a vaccine against COVID-19 yet, some people are hoping that this way we can achieve herd immunity. According to Gypsyamber D’Souza and David Dowdy, epidemiology professors at the Johns Hopkins Bloomberg School of Public Health, recovered COVID-19 patients may only be immune to the disease “for months to years”. We don’t currently have enough information to know how re-infection with this particular coronavirus works, but we do know that people can be re-infected with other coronaviruses. In other words, we shouldn’t expect to achieve herd immunity in this way.
In addition, it is estimated that at least 70 percent of people would need immunity to COVID-19 for the entire population to have herd immunity. Our healthcare system would quickly be overwhelmed by the number of cases and the catastrophic number of deaths that would occur if people were intentionally infected in order to gain immunity.
In summary, our best choices for conquering COVID-19 is social distancing, wearing a mask, and patiently waiting for a vaccine.
[h/t Johns Hopkins Bloomberg School of Public Health]